When data is transferred among a plurality of semiconductor devices or when data is transferred from a semiconductor device in response to a request from a data processor, parallel data in a semiconductor device is converted into serial data and the data is transferred in synchronization with a clock at high speed. For example, Joint Electron Device Engineering Council (JEDEC) Solid State Technology Association defines standards for various types of double data rate synchronous dynamic random access memory (DDR SDRAM). Based on such standards, a plurality of address data transferred in series from a memory controller is converted into parallel data in a memory device, and the parallel data is written in a memory. Also, a plurality of address data read in parallel from a memory is converted into serial data, and the data is transferred to a memory controller. Particularly, when parallel data is converted to serial data, a multiplexer is used.
Patent Document 1 discloses a memory device that converts data read from a memory in parallel into serial data at high speed. FIG. 3 of Patent Document 1 discloses a circuit including multiplexers each converting two data that is read from a memory and that is inputted in parallel into serial data. The multiplexers are connected in series in three stages to convert 8-bit parallel data into 8-bit serial data. Patent Document 1 does not disclose the configuration of these multiplexers. However, as disclosed in FIG. 14 of Patent Document 2, a 2-to-1 multiplexer using two transmission gates is generally used. Each of the transmission gates has an input node connected to data inputted in parallel and an output node connected in wired-OR. According to Patent Document 2, the two transmission gates are switched by an edge of an identical clock.    Patent Document 1:    Japanese Patent Kokai Publication No. JP-P2009-211800A, which corresponds to US Patent Application Publication No. US2009/0219764A1.    Patent Document 2:    Japanese Patent Kokai Publication No. JP-P2001-118382A, which corresponds to U.S. Pat. No. 6,335,889B1.